Certain current tires, referred to as “highway” tires, are intended to travel at high speed and on increasingly long journeys, owing to the improvement in road networks and the growth in motorway networks throughout the world. All the conditions under which such a tire is required to travel without doubt make it possible to increase the number of kilometres travelled, the wear of the tire being less; on the other hand, the endurance of the latter, and in particular of the crown reinforcement, is impaired.
The lack of endurance relates both to the resistance to fatigue of the crown plies, and in particular the resistance to separation between ends of plies, and the resistance to fatigue of the cables of the portion of carcass reinforcement located beneath the crown reinforcement, the first inadequacy being greatly influenced by the operating temperature prevailing at the edges of the working plies, be it when traveling in a straight line or during drift travel.
A first solution has been described in patent application WO96/20095 and proposes arranging, on one hand between the carcass reinforcement and the crown reinforcement working ply radially closest to the axis of rotation, an axially continuous ply, formed of inextensible metal cables forming an angle at least equal to 60° with the circumferential direction, and the axial width of which is at least equal to the axial width of the shortest working crown ply, and on the other hand between the two working crown plies, an additional ply formed of metallic elements which are oriented substantially parallel to the circumferential direction, the axial width of said ply being at least equal to 0.7 S0.
The problems relating to the separation between working plies and the resistance to fatigue of the carcass reinforcement cables are thus substantially reduced, and the operating temperatures are lowered; on the other hand prolonged travel of the tires thus constructed caused fatigue failure to appear in the cables of the additional ply and more particularly the edges of said ply, whether the so-called triangulation ply be present or not.
In order to overcome these new drawbacks mentioned above and to improve the endurance of the crown reinforcement of the type of tire in question, patent application WO99/24269, not published to date, proposes, on either side of the equatorial plane and in the immediate axial extension of the additional ply of reinforcement elements which are substantially parallel to the circumferential direction, to couple, over a certain axial distance, the two working crown plies formed of reinforcement elements crossed from one ply to the next, then to decouple them by means of profiled elements of rubber mix at least over the remainder of the width common to said two working plies.
In order to improve the endurance of the crown reinforcement of the type of tire in question, without being faced with problems of fatigue of reinforcement elements, patent application WO99/58350 radically modifies the orientation of the inextensible reinforcement elements of the additional ply arranged radially between said working plies, said elements then being radial.
The shearing stresses between the two working crown plies are very great and more particularly in the case of coupling of the two what are called working plies, which involves delamination between the plies as the tire becomes fatigued.
Patent application WO 99/58351 thus proposes, associated with an additional ply the orientation of the reinforcement elements of which is radial, coupling the two working crown plies in the immediate axial extension of the additional ply, these then being decoupled by profiled elements of rubber mix at least over the remainder of the width common to said two working plies.
In order to overcome the above drawbacks and to improve still further the endurance of the crown reinforcement of such tires, patent application WO 00/69659 proposes combining the advantages of radial orientation with those of circumferential orientation of the reinforcement elements of the additional ply located radially between the two working crown plies. According to this document, an additional reinforcement formed of at least one ply of reinforcement elements, which is arranged radially between two working plies, is axially composed of three parts: a central part in the form of a ply formed of metallic reinforcement elements which are inextensible and substantially radial, and two lateral parts in the form of strips each formed of elastic circumferential metallic reinforcement elements.
U.S. Pat. No. 4,237,953 furthermore describes crown architectures formed of two working plies, the axial ends of one of them being turned up to cover the axial ends of the other ply with the aim of increasing the longitudinal rigidity of these zones.
Cables are said to be inextensible when said cables have a relative elongation at most equal to 0.2% under a tensile force equal to 10% of the breaking load.
Cables are said to be elastic when said cables have a relative elongation at least equal to 4% under a tensile force equal to the breaking load.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction of rolling of the tire. Circumferential reinforcement elements are elements which form angles within the range +2.5°, −2.5° around 0° with said direction.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction intersecting and perpendicular to the axis of rotation of the tire. Substantially radial reinforcement elements are elements which form with the meridian direction angles within the range +5°, −5° around 0°.
The axis of rotation of the tire is the axis around which it rotates in normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential median plane, or equatorial plane, is a plane which is perpendicular to the axis of rotation of the tire and divides the tire into two halves.